1. Field of the Invention
The present invention relates to a receiving device, and more particularly, to a receiving device that receives data encoded with a Manchester code and a tire pressure monitoring system using the receiving device.
2. Description of the Related Art
In recent years, a tire pressure monitoring system (hereinafter, referred to as TPMS) mounted in a vehicle has been disclosed. In the TPMS, a transmitter having a built-in battery is installed in a tire, and tire pressure data transmitted from the transmitter is received by a receiver installed in a vehicle body. In the wireless communication, two kinds of modulation methods, such as FM or AM, are generally used. According to the FM method, since an electric wave is continuously transmitted, a consumed capacity in the battery of the transmitter is very large. In order to resolve this problem, if the transmission speed is increased so as to reduce the transmission time or the number of transmissions is reduced so as to decrease the consumed capacity of the battery, the reliability of the communication may be deteriorated. According to the AM method, only when an electric wave is transmitted, since a transmission circuit may be operated, the transmission circuit can be operated for a longer period of time, and the data transmitting speed or the number of the transmissions can be increased, as compared with the case in which the FM method is used.
As for a method of encoding the tire pressure data, a Manchester code method or an NRZ (Non Return to Zero) code method is generally used. In the Manchester code method, the bit data (logical value) is represented by the rising edge or falling edge of the pulse at the center of the bit. In addition, even if the same bit data (logical value) is continuous, since the H or L of the signal level is not continuous, the determination is easy, and the transmission circuit can be operated for a longer period of time, as compared with the NRZ code method. In addition, since the Manchester code method is a code method that has strong resistance against the noise, it is preferable. In the Manchester code method according to the related art, a decoding process is performed by determining whether a rising edge or a falling edge of a pulse exists between bits (for example, see JP-T-08-507735).
The tire pressure monitoring system has a structure having a transmission device mounted in a tire, a receiving device mounted in a vehicle body, and tire pressure data transmitted from the transmission device to the receiving device. In this structure, the ambient temperature of the transmitting device rapidly varies by the heat emitted from a brake or a tire, and a carrier wave frequency of a transmission electric wave, a pulse width, a rising or falling timing of a pulse, or the like vary. Due to this variation, the bit data may be erroneously determined in the receiving device, and thus it is not possible to perform the decoding process of the data with high reliability.